In a conventional optical disc system, to sense the position of the laser beam in relation to the track on the disc, a main laser beam creates a reflection from the disc. The reflection from the main laser beam is picked up by 4 photo-diode sensors within a photo-diode sensor assembly. FIG. 1 is a conceptual diagram illustrating how such a photo-diode sensor assembly 10 is laid out in relation to the track direction. The outputs of the 4 photo-diodes within the photo-diode assembly 10 (when the laser beam is focused on the disc) are shown as signals A, B, C and D, respectively.
Referring to FIG. 2, a conventional optical disc system 20 is shown. A focus actuator (not shown) will keep a laser beam 22 focused on a surface of the disc 23 by adjusting the vertical position of a lens 24. A focus controller (not shown) controls the focus actuator. To control the focus actuator to keep the laser beam 22 focused on the surface of the disc 23, a signal focus error (FE) is controlled to zero and a signal beam strength (BS) is controlled to a high value. The signal FE provides information related to the vertical position of the lens 24. The signal BS provides information related to the strength of the laser beam 22. Due to the alignment of the photo-diode sensor 10 when the signal FE is controlled to zero, the focus point of the laser beam 22 may not be optimally positioned on the surface of the disc 23. It is necessary to adjust the signal FE slightly off of a zero level in order to optimally establish the focus point of the laser beam 22 on the surface of the disc 23. Conventional methods fail to provide an optimal focus point of the laser beam on the surface of the disc when the signal FE is slightly off a zero level (or at an optimized focus offset level). Since conventional methods fail to provide an optimal focus point of the laser beam on the disc, the overall quality of reading and writing data from and to the disc will be decreased.
It would be desirable to provide a method and/or apparatus to optimize the focus point for an optical disc.